📚 Electrolysis: A-Level OCR Chemistry Key Points Revision | A-Level OCR 化学：电解 考点精讲
Electrolysis is the process of using direct electric current to drive a non-spontaneous chemical reaction. It is a fundamental topic in A-Level OCR Chemistry, involving the decomposition of electrolytes, electrode reactions, and quantitative calculations. Mastering electrolysis is essential for understanding industrial processes like aluminium extraction and electroplating.
电解是利用直流电驱动非自发化学反应的过程。这是 A-Level OCR 化学中的一个基础专题，涉及电解质的分解、电极反应和定量计算。掌握电解对于理解铝的提炼和电镀等工业过程至关重要。
1. Introduction to Electrolysis | 电解简介
Electrolysis involves using an external power source to force electrons through an electrolyte, causing redox reactions at the electrodes. The cathode is the negative electrode where reduction occurs, and the anode is the positive electrode where oxidation occurs. This contrasts with galvanic cells where the redox reaction is spontaneous.
电解涉及使用外部电源迫使电子通过电解质，从而在电极上引发氧化还原反应。阴极是发生还原反应的负极，阳极是发生氧化反应的正极。这与原电池中的自发氧化还原反应形成对比。
Electrolysis is used to decompose compounds into their elements. The electrolyte must be molten or dissolved in water to allow the free movement of ions that carry charge between the electrodes.
电解用于将化合物分解为单质。电解质必须是熔融态或溶于水，以便离子自由移动并在电极间传输电荷。
2. Electrolytic Cells vs Galvanic Cells | 电解池与原电池对比
In an electrolytic cell, electrical energy is converted to chemical energy, requiring a DC power supply. The anode is positive and the cathode is negative, opposite to a galvanic cell. Ions migrate: cations toward the cathode, anions toward the anode. Electrons flow from the negative terminal of the power source to the cathode, and from the anode back to the positive terminal.
在电解池中，电能转化为化学能，需要直流电源。阳极为正极，阴极为负极，与原电池相反。离子迁移：阳离子向阴极移动，阴离子向阳极移动。电子从电源负极流向阴极，再从阳极流回电源正极。
The key difference is that a galvanic cell produces electricity from a spontaneous redox reaction, while an electrolytic cell consumes electricity to force a non-spontaneous reaction.
关键区别在于：原电池通过自发的氧化还原反应产生电能，而电解池则消耗电能驱动非自发反应。
3. Electrolytes and Electrodes | 电解质与电极
Electrolytes are ionic compounds that conduct electricity when molten or dissolved in water, due to freely moving ions. Common electrolytes include molten NaCl, aqueous CuSO₄, and dilute H₂SO₄. Electrodes are typically inert (platinum, graphite) or active (copper, silver) depending on the process.
电解质是在熔融或溶于水时能够导电的离子化合物，因为离子可以自由移动。常见电解质包括熔融 NaCl、CuSO₄ 溶液和稀 H₂SO₄。电极通常是惰性的（铂、石墨）或活性的（铜、银），取决于工艺要求。
Inert electrodes do not participate in the reaction, whereas active electrodes can take part, e.g., a copper anode dissolves in copper refining.
惰性电极不参与反应，而活性电极可能会参与反应，例如铜精炼中铜阳极会溶解。
4. Ion Migration and Discharge | 离子迁移与放电
Under an applied potential difference, cations are attracted to the cathode to gain electrons (reduced), and anions move to the anode to lose electrons (oxidised). The discharge of ions depends on their position in the electrochemical series, their concentration, and the electrode material used.
在外加电场下，阳离子被吸引到阴极获得电子（被还原），阴离子移向阳极失去电子（被氧化）。离子的放电顺序取决于其在电化学系列中的位置、离子浓度及所使用的电极材料。
The following table summarises the usual discharge order for cations and anions in aqueous solutions using inert electrodes. Note that overpotential can affect the practical order, especially for gases.
下表总结了使用惰性电极时，水溶液中阳离子和阴离子的常见放电顺序。注意，过电位可能影响实际顺序，尤其是对气体产物。
| Cations (ease of discharge decreases downwards) | Anions (ease of discharge decreases downwards) |
|---|---|
| Ag⁺ | I⁻ |
| Cu²⁺ | Br⁻ |
| H⁺ (from water/acid) | Cl⁻ |
| Pb²⁺ | OH⁻ (from water/alkali) |
| Zn²⁺ | NO₃⁻ |
| Al³⁺ | SO₄²⁻ |
| Mg²⁺ | |
| Na⁺ | |
| K⁺ |
For cations above hydrogen in the reactivity series, water is reduced instead. For anions, halides discharge before hydroxide, but in very dilute halide solutions or with sulfate/nitrate electrolytes, oxygen is produced at the anode from the oxidation of water or hydroxide ions.
对于活泼性高于氢的阳离子，实际放电的是水。对于阴离子，卤素离子比氢氧根优先放电；但在极稀的卤离子溶液或硫酸盐/硝酸盐电解质中，阳极会析出氧气，来自水或氢氧根的氧化。
5. Predicting Products in Molten Electrolytes | 熔融电解质产物预测
In a molten electrolyte, only the ions from the salt are present, so the products are straightforward. For example, molten sodium chloride yields liquid sodium at the cathode and chlorine gas at the anode. Cathode: Na⁺ + e⁻ → Na(l); Anode: 2Cl⁻ → Cl₂(g) + 2e⁻.
在熔融电解质中，只存在盐自身的离子，因此产物直接明了。例如，熔融氯化钠在阴极产生液态钠，阳极产生氯气。阴极：Na⁺ + e⁻ → Na(l)；阳极：2Cl⁻ → Cl₂(g) + 2e⁻。
Other examples include molten lead(II) bromide: Pb²⁺ + 2e⁻ → Pb(l) and 2Br⁻ → Br₂(g) + 2e⁻. Aluminium oxide is also electrolysed in the molten state using cryolite to lower the melting point.
其他例子包括熔融溴化铅：Pb²⁺ + 2e⁻ → Pb(l) 和 2Br⁻ → Br₂(g) + 2e⁻。氧化铝也是以熔融态电解，并添加冰晶石以降低熔点。
6. Predicting Products in Aqueous Solutions | 水溶液中的产物预测
In aqueous solutions, water molecules provide H⁺ and OH⁻ that compete with solute ions for discharge. The product at each electrode depends on the E° values of all possible half-reactions. At the cathode, if the metal ion is less reactive than hydrogen (e.g., Cu²⁺, Ag⁺), it will be reduced to the metal. If the metal is more reactive (e.g., Na⁺, Mg²⁺), water is reduced to hydrogen gas: 2H₂O + 2e⁻ → H₂ + 2OH⁻ (in neutral/alkaline) or 2H⁺ + 2e⁻ → H₂ (in acidic).
在水溶液中，水分子提供的 H⁺ 和 OH⁻ 会与溶质离子竞争放电。各电极的产物取决于所有可能半反应的 E° 值。在阴极，如果金属离子的活泼性低于氢（如 Cu²⁺、Ag⁺），则金属被还原析出。若金属更活泼（如 Na⁺、Mg²⁺），则水被还原产生氢气：2H₂O + 2e⁻ → H₂ + 2OH⁻（中性/碱性）或 2H⁺ + 2e⁻ → H₂（酸性）。
At the anode, concentrated halide ions (Cl⁻, Br⁻, I⁻) are discharged as the halogen gas. In dilute solutions or with oxoanions (SO₄²⁻, NO₃⁻), hydroxide ions or water are oxidised to oxygen: 4OH⁻ → O₂ + 2H₂O + 4e⁻ (alkaline) or 2H₂O → O₂ + 4H⁺ + 4e⁻ (acidic). For example, electrolysis of aqueous CuSO₄ with inert electrodes gives copper at the cathode and oxygen at the anode, while the electrolyte becomes acidic due to H⁺ formation.
在阳极，浓卤离子（Cl⁻、Br⁻、I⁻）放电生成卤素单质。在稀溶液或含氧酸根离子（SO₄²⁻、NO₃⁻）环境中，氢氧根或水被氧化生成氧气：4OH⁻ → O₂ + 2H₂O + 4e⁻（碱性）或 2H₂O → O₂ + 4H⁺ + 4e⁻（酸性）。例如，用惰性电极电解 CuSO₄ 溶液时，阴极得到铜，阳极得到氧气，同时电解液因 H⁺ 生成而变酸性。
7. Standard Electrode Potentials and Selective Discharge | 标准电极电势与优先放电
The selective discharge at electrodes is governed by standard electrode potentials (E°). The half-reaction with the most positive E° is most easily reduced at the cathode, while the one with the most negative E° (or least positive) is most easily oxidised at the anode. For CuSO₄(aq), Cu²⁺(aq) + 2e⁻ → Cu(s) has E° = +0.34 V, which is more positive than 2H⁺(aq) + 2e⁻ → H₂(g) (E° = 0.00 V), so copper deposits at the cathode.
电极上的选择性放电由标准电极电势（E°）决定。E° 最正的半反应最易在阴极被还原，E° 最负（
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